The present invention relates to an ultrasonic flaw detection method and an instrument therefor. The present invention is specifically suitable for detecting internal flaws such as nonmetallic inclusions in a rolled metallic sheet including a steel sheet. By use of the present invention, flaw detection at a time of a linear region with definite width is possible.
An internal flaw such as microscopic nonmetallic inclusion of approximately 50 xcexcm in diameter in a rolled metallic sheet may cause a crack when the rolled metallic sheet is pressed or drawn. Therefore, it is required for the internal flaw inspection of a rolled metallic sheet to detect an extremely small internal flaw.
Generally, the ultrasonic flaw detection method is most frequently applied to internal flaw inspection of rolled metallic materials. In this method, ultrasonic waves are propagated into rolled metallic materials so as to detect discontinuity in ultrasound propagation caused by the internal flaw. As an applied example of this method, there is a method for flaw inspection of entire volume of the rolled metallic sheet at a transfer line of the rolled metallic sheet. In Japanese Unexamined Patent Publication No. 7-253414, for example, the following ultrasonic flaw detection method and the instrument therefor are proposed. That is, in medium, a line-focused ultrasonic transmitting probe and a linear probe array are arranged face to face with a sheet being inspected (a rolled metallic sheet) between them. A line-focused ultrasonic beam transmitted from the transmitting probe propagates into the sheet being inspected approximately in a perpendicular direction thereto, so that part of ultrasound reflected at an internal flaw in the sheet being inspected will be received by the linear probe array. After the ultrasonic signal which had been received and transformed into the electrical signal was amplified and only the echo from internal flaw was picked up therefrom, any signal greater than a predetermined threshold voltage is detected.
However, in order to detect flaws effectively by use of the above-mentioned ultrasonic flaw detection method and the instrument therefor, the gap length xe2x80x9cLsxe2x80x9d (mm) between the line-focused ultrasonic transmitting probe and the linear probe array is required to satisfy the following equation. In this equation, xe2x80x9cFxe2x80x9d (mm) represents a focal length in medium, of the line-focused ultrasonic transmitting probe, and xe2x80x9ctxe2x80x9d (mm) denotes a thickness of the sheet being inspected.
Lsxe2x89xa6Fxe2x88x92{(CS/CW)xe2x88x921}t+5.5
(Provided that: xe2x80x9cCSxe2x80x9d; ultrasonic velocity (m/sec) in the sheet being inspected, xe2x80x9cCWxe2x80x9d; ultrasonic velocity (m/sec) in the medium) Accordingly, when a steel sheet of 4.5 mm in thickness is inspected and the focal length in the medium of the line-focused ultrasonic transmitting probe xe2x80x9cFxe2x80x9d=38 mm, the gap length xe2x80x9cLsxe2x80x9d between the transmitting probe and the receiving probe is required to be less than 31 mm.
A problem with this method is that there may be cases that the sheet being inspected in on-line inspection has a wavy shape in its edge or side portion. When the sheet having such a wavy shape is passed through between the transmitting probe and the receiving probe with the gap length of less than 31 mm, it may frequently hit the housing of the probe to be scratched thereon. The impact of the hit on the probe shortens probe""s useful life. In the worst case, the probe is broken.
It is an object of the present invention to provide an ultrasonic flaw detection method and an instrument therefor, having such an enough gap length between the transmitting probe and the receiving probe to be passed through by the sheet being inspected having a wavy shape that the sheet will not hit the probes and moreover being reliably detectable the internal flaw such as a microscopic nonmetallic inclusion.
The inventors have ardently studied conventional ultrasonic flaw detection methods, so that the present invention has been made based on a new knowledge that the gap length between a line-focused ultrasonic transmitting probe and a line-focused ultrasonic receiving probe is determined by the height of a flaw echo which is a function of a focal length in a coupling medium of the line-focused ultrasonic beam of the line-focused ultrasonic transmitting probe and a focal length in a coupling medium of the line-focused receiving beam of the line-focused ultrasonic receiving probe, and so forth. That is, summarized configurations of the present invention are as follows.
(1) An ultrasonic flaw detection method comprising the steps of: transmitting ultrasonic waves into the sheet being inspected approximately in a perpendicular direction to the sheet through a coupling medium with a line-focused ultrasonic transmitter; receiving ultrasonic waves reflected at an internal flaw through the coupling medium with a line-focused ultrasonic receiver; amplifying the received ultrasonic signals which have been transformed into electrical signals; picking up amplified signals of the echo from the internal flaw; and detecting the flaw by detecting the signal more than a predetermined threshold amplitude, wherein the transmitter and the receiver are arranged face to face with the sheet being inspected between them, and wherein the gap length (L) between the transmitter and the receiver is near the minimum value (Lp) in which the height (f(L))of the echo from the internal flaw takes the maximum value.
(2) The method in the above (1), wherein Lp is determined by a focal length (FT) of the line-focused ultrasonic transmitter in the coupling medium, a focal length (FR) of the line-focused ultrasonic receiver in the coupling medium, the velocity (CS) of ultrasonic waves in the sheet being inspected, the velocity (CW) of ultrasonic waves in the coupling medium, and the thickness (t) of the sheet being inspected.
(3) The method in the above (2), wherein when Lp1 and Lp2 (Lp1 less than Lp2) are the values of L in which f(L) gives f(L)/f(Lp)=xe2x88x923 dB, L is more than Lp1 and less than Lp2.
(4) The method in the above (2), wherein the coupling medium is a liquid, and wherein Lp satisfies Lp=0.75(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t.
(5) The method in the above (3), wherein the coupling medium is a liquid, and wherein Lp satisfies Lp=0.75(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t.
(6) The method in the above (5), wherein Lp1 and Lp2 satisfy Lp1=0.68(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t, Lp2=0.81(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t, respectively.
(7) An ultrasonic flaw detecting instrument comprising: a line-focused ultrasonic transmitter transmitting ultrasonic waves into the sheet being inspected approximately in a perpendicular direction to the sheet through a coupling medium; a line-focused ultrasonic receiver receiving ultrasonic waves reflected at an internal flaw through the coupling medium; a receiving amplifier amplifying the received ultrasonic signals which have been transformed into electrical signals; a gating means for picking up amplified signals of the echo from the internal flaw; and a comparator detecting the signals of the echo from the internal flaw which is more than or equal to a predetermined threshold amplitude, wherein the transmitter and the receiver are arranged face to face with the sheet being inspected between them, and wherein the gap length (L) between the transmitter and the receiver is near the minimum value (Lp) in which the height (f(L)) of the echo from the internal flaw takes the maximum value.
(8) The instrument in the above (7), wherein Lp is determined by a focal length (FT) of the line-focused ultrasonic transmitter in the coupling medium, a focal length (FR) of the line-focused ultrasonic receiver in the coupling medium, the velocity (CS) of ultrasonic waves in the sheet being inspected, the velocity (CW) of ultrasonic waves in the coupling medium, and a thickness (t) of the sheet being inspected.
(9) The instrument in the above (8), wherein when Lp1 and Lp2 (Lp1 less than Lp2) are the values of L in which f(L) gives f(L)/f(Lp)=xe2x88x923 dB, L is more than Lp1 and less than Lp2.
(10) The instrument in the above (8), wherein the coupling medium is a liquid, and wherein Lp satisfies Lp=0.75(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t.
(11) The instrument in the above (9), wherein the coupling medium is a liquid, and wherein Lp satisfies Lp=0.75(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t.
(12) The instrument in the above (11), wherein Lp1 and Lp2 satisfy Lp1=0.68(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t, Lp2=0.81(FT +FR)xe2x88x92{(CS/CW)xe2x88x921}t, respectively.
(13) The instrument in any one of the above items (7) to (12), the line-focused ultrasonic transmitter and the line-focused ultrasonic receiver are linear probe arrays respectively.
Referring now to the drawings, the present invention will be described in detail.
FIG. 1 shows a basic configuration of the present invention. A line-focused ultrasonic transmitter 20 and a line-focused ultrasonic receiver 30 are arranged face to face with a rolled metallic sheet 10 being inspected between them. Coupling medium such as water exists between the line-focused ultrasonic transmitter 20 and the sheet 10 being inspected, and between the line-focused ultrasonic receiver 30 and the sheet 10 being inspected. As for the line-focused ultrasonic transmitter 20 and the line-focused ultrasonic receiver 30, a line-focused single-element probe or a line-focused linear (one-dimensional) probe array may be used. FIG. 2 illustrates positional relationship between the line-focused ultrasonic transmitter 20 and the sheet 10 being inspected, and between the line-focused ultrasonic receiver 30 and the sheet 10 being inspected. The focal length of a line-focused ultrasonic beam 21 transmitted from the line-focused ultrasonic transmitter 20 in a coupling medium is denoted as xe2x80x9cFTxe2x80x9d (mm). The focal length of a line-focused receiving beam 31 formed by the line-focused ultrasonic receiver 30 in a coupling medium is denoted as xe2x80x9cFRxe2x80x9d (mm). At this time, the gap length xe2x80x9cLxe2x80x9d (mm) between the line-focused ultrasonic transmitter 20 and the line-focused ultrasonic receiver 30 is set so as to satisfy the equation (1). This is the feature of the present invention.
0.68(FT+FR)xe2x89xa6L+{(CS/CW)xe2x88x921}txe2x89xa60.81(FT+FR)xe2x80x83xe2x80x83(1)
Provided that:
CS: the ultrasonic velocity in the rolled metallic sheet (m/sec),
CW: the ultrasonic velocity in the coupling medium (m/sec),
t: the thickness of the rolled metallic sheet (mm).
In Japanese Unexamined Patent Publication No. 7-253414, on the basis of an experimental result, when the focal length in the coupling medium of the line-focused beam transmitted from the ultrasonic transmitter is denoted as xe2x80x9cFxe2x80x9d (mm), the gap length xe2x80x9cLsxe2x80x9d (mm) between the ultrasonic transmitter and the ultrasonic receiver is set so as to satisfy the equation (2). This is based on the result of the experiment in which, using the line-focused ultrasonic transmitter with the focal length xe2x80x9cFxe2x80x9d=38 mm and water as the coupling medium, heights of echoes from internal flaws are measured by changing xe2x80x9cLsxe2x80x9d from 10 mm to 35 mm approximately.
Lsxe2x89xa6Fxe2x88x92{(CS/CW)xe2x88x921}t+5.5xe2x80x83xe2x80x83(2)
On the requirement of the equation (2), the contact accident of the sheet being inspected with the ultrasonic transmitter and the ultrasonic receiver cannot be avoided as described above.
Therefore, the inventors ardently studied the method and the instrument therefor to further increase the gap length between the ultrasonic transmitter and the ultrasonic receiver for preventing the contact accident. As a result of the study, it has been definitely shown by the present invention that the gap length between the ultrasonic transmitter and the ultrasonic receiver can be increased further, while moreover the echoes from internal flaws can be effectively received.
The experiment leading to the invention will be described.
With an instrument comprising a line-focused ultrasonic transmitter with a focal length in water xe2x80x9cFTxe2x80x9d being 38 mm, and a line-focused ultrasonic receiver with a focal length in water xe2x80x9cFRxe2x80x9d being 38 mm, the relationship between the height of the echo from an internal flaw and the gap length xe2x80x9cLxe2x80x9d between the line-focused ultrasonic transmitter and the line-focused ultrasonic receiver is surveyed, using water as the coupling medium. A steel sheet of 4.5 mm in thickness having an internal flaw of 50 xcexcm in width and 100 xcexcm in length is used as the sheet being inspected. The result thereof is shown in FIG. 3.
The height xe2x80x9cf(L)xe2x80x9d of the echo from the internal flaw decreases as xe2x80x9cLxe2x80x9d increases in the range of xe2x80x9cLxe2x80x9dxe2x89xa635 mm. The xe2x80x9cf(L)xe2x80x9d, however, rises quickly in the range of xe2x80x9cLxe2x80x9d greater than 35 mm to take the maximum value at xe2x80x9cLxe2x80x9d=43 mm, falls again thereafter. In the vicinity of the maximum value, the sufficient xe2x80x9cf(L)xe2x80x9d is secured.
The gap length xe2x80x9cLpxe2x80x9d between the line-focused ultrasonic transmitter and the line-focused ultrasonic receiver in which the height xe2x80x9cf(L)xe2x80x9d of the echo from the internal flaw takes the maximum value is given by the equation (3).
Lp=0.75(FT+FR)xe2x88x92{(CS/CW)xe2x88x92}txe2x80x83xe2x80x83(3)
Provided that;
FT: the focal length (mm) in the coupling medium of the line-focused beam transmitted by the line-focused ultrasonic transmitter,
FR: the focal length (mm) in the coupling medium of the line-focused beam formed by the line-focused ultrasonic receiver,
CS: the ultrasonic velocity (m/sec) in the sheet being inspected,
CW: the ultrasonic velocity (m/sec) in the coupling medium,
t: the thickness (mm) of the sheet being inspected.
This relationship has been found by the following experiment. Three kinds of line-focused ultrasonic transmitters and of line-focused ultrasonic receivers, each kind having the focal length xe2x80x9cFTxe2x80x9d or xe2x80x9cFRxe2x80x9d in the coupling medium of 38 mm, 57 mm, and 76 mm, respectively, are prepared. Using water as the coupling medium, under the combination shown in Table 1, the same experiment as described above has been carried out to obtain the gap length xe2x80x9cLpxe2x80x9d at which the height of the echo from the internal flaw takes the maximum value. The result thereof is shown in FIG. 4. The horizontal axis represents the sum of the focal length xe2x80x9cFTxe2x80x9d of the line-focused ultrasonic transmitter and the focal length xe2x80x9cFRxe2x80x9d of the line-focused ultrasonic receiver (FT+FR). The sum (FT+FR) and the minimum value of the gap length xe2x80x9cLpxe2x80x9d in which the height of the echo from the internal flaw takes the maximum value are linearly correlated and the slope thereof is 0.75. Accordingly, the minimum value of the gap length xe2x80x9cLpxe2x80x9d in which the height of the echo from the internal flaw takes the maximum value can be given by the equation (4).
Lp=0.75(FT+FR)+xcex1(provided that xe2x80x9cxcex1xe2x80x9d is a constant)xe2x80x83xe2x80x83(4)
The value of the constant xe2x80x9cxcex1xe2x80x9d has been studied as follows:
The value xe2x80x9cxcex1xe2x80x9d is obtained from FIG. 4 to be approximately xe2x88x9213.4 (mm). It is estimated that the focal lengths xe2x80x9cFTxe2x80x9d and xe2x80x9cFRxe2x80x9d are observed to be reduced by refraction of the beam, when the sheet being inspected is located within the transmitting line-focused beam and the receiving line-focused beam. This effect can be expressed by obtaining the value xe2x80x9cxcex1xe2x80x9d from the equation (5).
xcex1=xe2x88x92{(CS/CW)xe2x88x921}txe2x80x83xe2x80x83(5)
CS=5950 m/sec for the steel sheet, CW=1500 m/sec for water at room temperature, and the steel sheet thickness for the experiment is 4.5 mm. Using these values, the right side of the equation (5) is calculated to obtain the value of xe2x88x9213.35 (mm), which quite coincides with the experimental result.
Therefore, the minimum value of the gap length xe2x80x9cLpxe2x80x9d in which the height of the echo from the internal flaw takes the maximum value can be arranged to be the equation (3).
Practically, if the echo height is within xe2x88x923 dB with reference to the maximum value flaw detection with sufficient signal to noise ratio can be achieved. Based on this value, allowable range of the gap length xe2x80x9cLxe2x80x9d are obtained. From FIG. 3, the range of the gap length xe2x80x9cLxe2x80x9d in which the echo height is within xe2x88x923 dB with reference to the maximum value is obtained to be 38 to 48 mm.
As described above, the relationship between the height of the echo from the internal flaw and the gap length xe2x80x9cLxe2x80x9d depends on the focal length xe2x80x9cFTxe2x80x9d in the coupling medium of the line-focused beam transmitted by the line-focused ultrasonic transmitter and the focal length xe2x80x9cFRxe2x80x9d in the coupling medium of the line-focused beam formed by the line-focused ultrasonic receiver. Therefore, the allowable range of the gap length xe2x80x9cLxe2x80x9d can be also given by multiplying the sum of the focal length xe2x80x9cFTxe2x80x9d of the line-focused ultrasonic transmitter and the focal length xe2x80x9cFRxe2x80x9d of the line-focused ultrasonic receiver (FT+FR) by a coefficient.
The minimum of the allowable range of xe2x80x9cLxe2x80x9d is defined by xe2x80x9cLp1xe2x80x9d, while the maximum is represented by xe2x80x9cLp2xe2x80x9d. Provided that xe2x80x9cLp1xe2x80x9d=38 mm, the coefficient is calculated to be 0.68, and if xe2x80x9cLp2xe2x80x9d=48 mm, the coefficient is calculated to be 0.81. Therefore, Lp1 and Lp2 can be given by the following equation.
Lp1=0.68(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t
Lp2=0.81(FT+FR)xe2x88x92{(CS/CW)xe2x88x921}t
Accordingly, the allowable range of the gap length xe2x80x9cLxe2x80x9d can be given as follows.
0.68(FT+FR)xe2x89xa6L+{(CS/CW)xe2x88x921}txe2x89xa60.81(FT+FR)